Why do authors use pathetic fallacy?
They use it as a form of emphasis and drams.
In using pathetic fallacy, authors are introducing a physical dimension which emphasises the mood or tone they wish to express. For example in Macbeth, the references to the weather by Macbeth, the description of the witches and the bleakness of the landscape add a backdrop to the mood of the play which is dark.
Dickens in Great Expectations again uses it in the first chapters to paint a picture symbolic of the novel. The bleak marshes and the sinister figure of Magwitch enforce the harshness of the life of Pip and Joe Gargery.
The Brontes also used the harsh landscape of the Yorkshire moors in books such as Jane Eyre and Wuthering Heights to emphasise the harsh aspects of life faced by characters such as Jane in her experiences at Lowood school and the traumatic relationship with and dark past of Mr Rochester.